1. Field of the Invention
The present invention relates to overcoat methods for improving cleanability of high reflectance optical mirrors and, in particular, mirrors adapted for use in ring laser gyros.
2. Discussion of the Prior Art
To produce the very high reflecting mirrors needed for ring laser gyros, generally a number of optical regions of different refractive index materials are deposited onto suitable substrates. With a sufficient number of such optical regions, each appropriately thick, the reflectance of the resulting mirrors may be made acceptably high.
Conventional methods of fabricating these dielectric mirror stacks using typical sputtering techniques results in abrupt interfaces between each alternating optical region. In ion beam sputtering for example, ions are accelerated from a region in which they are generated (the ion beam source) by means of suitably electrified grids, and directed at high energies onto a target. On impact, material from the target is sputtered off, and subsequently received by suitably located substrates, resulting in the deposition of target material. These conventional procedures employ rapid transfers from one target material to another, interrupting the ion beam during target changes, or both. Such practices serve to increase the abruptness of the discontinuity of refractive index between layers.
U.S. Pat. No. 4,108,751, issued Aug. 27, 1978 to King, entitled "Ion Beam Implantation-Sputtering", discloses an ion beam apparatus wherein materials are deposited onto a substrate using a target which may be comprised of dual materials. King is directed toward a method of depositing a hard coating on a plastic lens. King angles the boundary of two materials on a target such that a continuous change in amounts of materials deposited can be attained as the location of the primary beam on the target is changed. King states that the target can be moved relative to the location of the primary ion beam at a speed necessary to make a transition region.
U.S. Pat. No. 5,240,583, issued Aug. 31, 1993 to Ahonen, entitled "Apparatus To Deposit Multilayer Films", assigned to the same assignee as the instant application, discloses an apparatus to deposit thin films on a substrate. An ion beam produced by an ion gun which is radio frequency excited impinges upon a target. The target is translatable laterally by a target holder to bring different target materials into contact with the ion beam. The targets are held at an angle to the ion beam. The target material sputtered off the target by collision with the ions is deposited on substrates held above the targets. The substrates are moved into and out of the path of the sputtered material by a rotating device for uniform deposition of the sputtered material.
Ring laser gyro mirrors must maintain low absorption loss despite long exposures to plasma environments used to provide lasing action in, for example, ring laser gyros. Resistance to plasma-induced increases of absorption loss may be achieved by mirrors composed of multiple optical regions of zirconia (ZrO.sub.2) and silica (SiO.sub.2), where the ZrO.sub.2 material is doped with a well-known glass forming material such as a small percentage of SiO.sub.2 to minimize crystallization during deposition and subsequent post deposition annealing operations. A high plasma resistance of such a mirror is obtained when the last material, that is, the outermost one, comprises ZrO.sub.2, and not SiO.sub.2. But such ZrO.sub.2 -topped mirrors, as conventionally made, possess surface properties which make them very difficult to clean. In particular, very small particulate contamination poses significant cleaning problems for such mirrors. It is well known that large amounts of such surface particulates are detrimental to ring laser gyro performance.
FIG. 1 shows the outermost part of a conventional mirror. FIG. 1 shows a high reflectance mirror 5 of the type employed in laser devices such as, for example, ring laser gyros. Those skilled in the art will recognize a typical configuration of zirconium oxide (ZrO.sub.2) 10 over a mirror structure 12 comprising, for example, silicon dioxide (SiO.sub.2). Those skilled in the art will also recognize that, while several of the optical quarterwave structures are shown, this is for illustrative purposes only and that many more alternating regions may be deposited and are typically deposited on such mirrors. Deposition of the alternating optical regions may be accomplished by any well-known means such as by electron beam deposition processes or ion beam deposition. The multiple region mirror 5 structure is topped with a final coating of ZrO.sub.2 10 having a surface 20 with no overcoat thereon.
FIG. 2a shows a multiple optical region mirror 15, with the addition of a thin coating 22 of SiO.sub.2 on the top surface 20 of the final ZrO.sub.2 coating 10. Such mirrors have been made by Honeywell Incorporated of Minneapolis, Minn. USA with an overcoat thickness of less than 150 angstroms and at least about 100 angstroms. This SiO.sub.2 overcoat is not thick enough to appreciably alter the optical properties of the structure, but returns the surface to the cleaning behavior of conventional glass. Unfortunately, it has been learned that mirrors such as mirror 15 shown in FIG. 2a may sometimes suffer from a lack of plasma resistance if the thickness of the overcoat region 22 of SiO.sub.2 is equal to or greater than about 100 angstroms. Also such mirrors do not exhibit the desired long life characteristics of mirrors made without such an overcoat surface.